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                <h2 id="宏打印函数"><a href="#宏打印函数" class="headerlink" title="宏打印函数"></a>宏打印函数</h2><p>在我们的嵌入式开发中，使用printf打印一些信息是一种常用的调试手段。但是，在打印的信息量比较多的时候，就比较难知道哪些信息在哪个函数里进行打印。</p>
<p>特别是对于异常情况的打印，我们需要快速定位到异常情况的位置。这时候我们可以使用宏定义来封装一个宏打印函数，这个宏打印函数可以显示打印信息所在的文件、行数、函数名等信息。如：</p>
<pre class=" language-c"><code class="language-c"><span class="token macro property">#<span class="token directive keyword">define</span> DBG_PRINTF(fmt, args...)  \
{\
    printf("&lt;&lt;File:%s  Line:%d  Function:%s>> ", __FILE__, __LINE__, __FUNCTION__);\
    printf(fmt, ##args);\
}</span></code></pre>
<p>使用范例：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQ39A.png" alt></p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQMAe.png" alt></p>
<p>可见，使用方法与printf的使用方法一样，而且每条打印语句开头都会打印调试信息所在的文件名、行号、函数名信息，方便我们查找一些调试信息。</p>
<p>其中，<code>__FILE__</code>、<code>__LINE__</code>、<code>__FUNCTION__</code>这三个宏是编译器内置宏定义，分别代表调试信息所在文件、行号、函数。除此之外，常用的宏还有：<code>__DATE__</code>、<code>__TIME__</code>，分别代表当前的编译日期与时间。如：</p>
<pre><code>DBG_PRINTF(&quot;Compile Time: %s  %s\n&quot;, __DATE__, __TIME__);</code></pre><p><img src="https://s1.ax1x.com/2020/05/17/YgQscq.png" alt></p>
<p>第二条printf中的##符号是为了处理args不代表任何参数的情况。如：</p>
<pre><code>DBG_PRINTF(&quot;Hello world&quot;);</code></pre><p>当不加##符号是，以上宏的第二条语句被拓展为：</p>
<pre class=" language-c"><code class="language-c"><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"Hello world\n"</span><span class="token punctuation">,</span> <span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre>
<p>可见，多出了一个逗号，这个逗号是多余的。</p>
<p>加上##符号后，以上宏的第二条语句被拓展为：</p>
<pre class=" language-c"><code class="language-c"><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"Hello world\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre>
<p>这才是我们想要的结果。其实这些结果我们通过查看预处理文件可以清晰的知道：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQu7D.png" alt></p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQyj0.png" alt><br><img src="https://s1.ax1x.com/2020/05/17/YgQcuV.png" alt></p>
<p>最后需要注意的是，这个DBG_PRINTF还是与printf不一样的。DBG_PRINTF宏是两条语句的组合，无返回值；而printf的原型是：</p>
<pre class=" language-c"><code class="language-c"><span class="token keyword">int</span> <span class="token function">printf</span> <span class="token punctuation">(</span><span class="token keyword">const</span> <span class="token keyword">char</span> <span class="token operator">*</span>__format<span class="token punctuation">,</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre>
<p>但是我们一般都很少使用printf的返回值，所以DBG_PRINTF的用法与printf函数基本一致。</p>
<h2 id="打印调试宏开关"><a href="#打印调试宏开关" class="headerlink" title="打印调试宏开关"></a>打印调试宏开关</h2><p>通常情况下，一些打印调试信息只是在我们调试阶段需要的，在程序发布阶段是不需要的。所以，为了避免打印调试信息带来的资源开销，我们可以把这些打印调试语句给注释掉。</p>
<p>一种方法是逐句进行注释，这是一种比较低效的方法。比较高效的方法就是添加调试宏开关，利用条件编译来选择打印/不打印调试信息。比如我们可以把上面的代码改造为：</p>
<pre class=" language-c"><code class="language-c"><span class="token macro property">#<span class="token directive keyword">define</span>  DEBUG   1  </span>

<span class="token macro property">#<span class="token directive keyword">if</span> DEBUG</span>
  <span class="token macro property">#<span class="token directive keyword">define</span> DBG_PRINTF(fmt, args...)  \
  {\
    printf("&lt;&lt;File:%s  Line:%d  Function:%s>> ", __FILE__, __LINE__, __FUNCTION__);\
    printf(fmt, ##args);\
  }</span>
<span class="token macro property">#<span class="token directive keyword">else</span></span>
  <span class="token macro property">#<span class="token directive keyword">define</span> DBG_PRINTF(fmt, args...)   </span>
<span class="token macro property">#<span class="token directive keyword">endif</span></span></code></pre>
<p>根据DEBUG宏的值来选择对应的打印宏函数。当DEBUG的值为1时启动相关的打印调试语句，DEBUG的值为0时则关闭打印调试语句。</p>
<p>这样我们就可以很方便的通过设置DEBUG宏的值来启动与关闭我们整个工程的DBG_PRINTF打印调试信息。</p>
<h2 id="do-while-0"><a href="#do-while-0" class="headerlink" title="do{}while(0)"></a>do{}while(0)</h2><p>其实，上面我们封装的打印宏DBG_PRINTF还有一点缺陷，比如我们与if、else使用的时候，会有这样的一种使用情况：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQn0O.png" alt></p>
<p>此时会报语法错误。为什么呢？</p>
<p>同样的，我们可以先来看一下我们的demo代码预处理过后，相应的宏代码会被转换为什么。如：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQQtH.png" alt></p>
<p>这里我们可以看到，我们的if、else结构代码被替换为如下形式：</p>
<pre class=" language-c"><code class="language-c"><span class="token keyword">if</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span>
<span class="token punctuation">{</span> <span class="token comment" spellcheck="true">/* ....... */</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">else</span>
<span class="token punctuation">{</span> <span class="token comment" spellcheck="true">/* ....... */</span> <span class="token punctuation">}</span><span class="token punctuation">;</span></code></pre>
<p>显然，出现了语法错误。if之后的大括号之后不能加分号，这里的分号其实可以看做一条空语句，这个空语句会把if与else给分隔开来，导致else不能正确匹配到if，导致语法错误。</p>
<p>为了解决这个问题，有几种方法。第一种方法是：把分号去掉。代码变成：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQ81I.png" alt>     </p>
<p>第二种方法是：在if之后使用DBG_PRINTF打印调试时总是加{}。代码变成：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQGct.png" alt></p>
<p>以上两种方法都可以正常编译、运行了。</p>
<p>但是，我们C语言中，每条语句往往以分号结尾；并且，总有些人习惯在if判断之后只有一条语句的情况下不加大括号；而且我们创建的DBG_PRINTF宏函数的目的就是为了对标printf函数，printf函数的使用加分号在任何地方的使用都是没有问题的。</p>
<p>基于这几个原因，我们有必要再对我们的DBG_PRINTF宏函数进行一个改造。</p>
<p>下面引入do{}while(0)来对我们的DBG_PRINTF进行一个简单的改造。改造后的DBG_PRINTF宏函数如下：</p>
<pre><code>#define DBG_PRINTF(fmt, args...)  \
do\
{\
    printf(&quot;&lt;&lt;File:%s  Line:%d  Function:%s&gt;&gt; &quot;, __FILE__, __LINE__, __FUNCTION__);\
    printf(fmt, ##args);\
}while(0)</code></pre><p>这里的do…while循环的循环体只执行一次，与不加循环是效果一样。并且，可以避免了上面的问题。预处理文件：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQJjP.png" alt></p>
<p>我们的宏函数实体中，while(0)后面不加分号，在实际调用时补上分号，既符合了C语言语句分号结尾的习惯，也符合了do…while的语法规则。</p>
<p>使用do{}while(0)来封装宏函数可能会让很多初学者看着不习惯，但必须承认的是，这确确实实是一种很常用的方法。</p>
<p>在STM32的HAL库中搜索while(0)：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQtnf.png" alt></p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQNB8.png" alt></p>
<p>在Linux源码中搜索while(0)：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQUHS.png" alt></p>
<p>可见，在实际中，do{}while(0)用的很多。</p>
<h2 id="运算符与-运算符"><a href="#运算符与-运算符" class="headerlink" title="#运算符与##运算符"></a>#运算符与##运算符</h2><p>这两个运算符之前也有分享过，这里顺便也提一下。</p>
<p><code>#号</code>作为一个<code>预处理运算符</code>，可以<code>把记号转换成字符串</code>。例如，如果A是一个宏形参，那么#A就是转换为字符串”A”的形参名。这个过程称为<code>字符串化（stringizing）</code>。以下程序演示这个过程：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQdAg.png" alt><br><img src="https://s1.ax1x.com/2020/05/17/YgQwNQ.png" alt></p>
<p>##运算符可以把两个记号组合成一个记号。以下程序演示这个过程：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQ0hj.jpg" alt><br><img src="https://s1.ax1x.com/2020/05/17/YgQD9s.png" alt></p>
<p>这个运算符用得很多。如：</p>
<p><img src="https://s1.ax1x.com/2020/05/17/YgQr3n.png" alt></p>
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                            常规打印方法在STM32的应用中，我们常常对printf进行重定向的方式来把打印信息printf到我们的串口助手。在MDK环境中，我们常常使用MicroLIB+fputc的方式实现串口打印功能，即：


要实现fputc函数的原因是：pri
                        
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#error指令#erro
                        
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